High-strength low-alloy steel

ABSTRACT

High strength low alloy steels having lower yield strengths in the range of from 50,000 psi to 80,000 psi and superior formability properties are disclosed.

United States Patent 1191 Creswick et al.

[ Dec. 16, 1975 HIGH-STRENGTH LOW-ALLOY STEEL 3,619,303 11/1971 Semel 148/36 3,666,570 5/1972 Knrchynsky et al. 148/36 [75] Inventors- Creswlck, m 3,711.340 1/1973 Korchynsky etal. 148/36 Hum, both of Sault Same Mane 3,721.587 3/1973 Allten et al. 148/36 Canada [73] Asslgnee: l fi a g gg g gg g Primary ExaminerW. Stallard r na a Attorney, Agent, or Firm-Browne, Beveridge, [22] Filed: Jan. 9 1974 DeGrandi & Kline [21] Appl. No.: 432,030

52 us. 01. 148/12F [57] ABSTRACT 2 [221g] LntidCLf ..l.4(2ll2DFl/g2 High Strength low alloy Steels having lower yield 1 le 0 earc strengths in the range of from 50,000 psi to 80,000 psi References Cited and superior formablllty properties are dlSC1OSCl.

UNITED STATES PATENTS 4 Claims, 2 Drawing Figures 3,102,831 9/1963 Tisdale 148/12 F .lO.l2 Cb TOTAL YIELD STRENGTH AS A FUNCTION 1 80.000 OF GRAIN REFINEMENT a PRECIPITATION STRENGTHENING i 5:25.300 70,000 E I z i I '5 03-04 Cb 1 5 0 000 I A$=22,00Q c1: 1 Total Lower Yield l Strength O l s I 50,000 01-02 a: l g A$=I0.600 c- .06 max 0 MrI-.30.89 40,000 Cb -..oI-.I2

Groin Refinement AS -Prec1pl1utlon Increase Slrenqthenlnq 8 9 IO 10.5 ll 11.5 12 l2.5 l3

GRAIN SIZE ASTM HIGH-STRENGTH LOW-ALLOY STEEL The present invention relates to high strength low content directly however more important the low carbon content and low manganese values act to increase the austenite to ferrite transformation temperature. It has been found that this increase or higher austenite to alloy steels and particularly to a series of high strength 5 ferrite transformation temperature controls the proporsteels having lower yield strengths in therange of from tion of the columbium that precipitates into coarse and 50,000 psi to 80,000 psi and having superior formabilfine dispersion in the composition and distributes the ity properties than steels presently available. available columbium into its dual form for the purposes There is an ever present and increasing demand for of grain refinement and precipitation strengthening. high strength steels having good formability properties The coarse precipitation of columbium occursduring particularly drawings, biaxial stretching and uniaxial' the hot deformation or actual rolling of the steel up to bending properties required, for example by the autoand including the final deformation, the collecting. motive industry for automobile bumper systems. This coarse precipitation acts to retard the recrystalli- The process of hot strip rolling as well as the manner zation of austenite, immediately after the final deforof controlling the temperature in such process is well mation, until transformation to ferrite is started. Transknown in the art thus it is not believed necessary to formation from the highly deformed austenite guaranprovide any specific details with respect to finishing or tees transformation to a fixed and constant ferrite grain collecting stages. The present invention is concerned size. The ferrite grain size will vary according to the only with the finishing and coiling temperatures of a amount of columbium present and the temperature at specific steel composition and the manner of attaining which the finishing rolling is carried out. This in turn such temperature control would be an obvious expediestablishes the final austenite grain size prior to transent to one skilled in the art. According to the present formation. After transformation from the austenite to invention there is provided a high strength low alloy the ferrite structure, the fine precipitation from the steel strip having a minimum yield strength of 50,000 balance of the columbium present will occur in the psi and good formability properties, said steel consistfully transformed ferrite. The grain size and spacing of ing essentially of, by weight, 0.10 percent maximum the fine precipitate will depend on the extent of cooling carbon, 0.30 to 0.80 percent manganese, 0.02 percent (spray quenching) and the coiling or collecting tempermaximum sulphur, 0.02 to 0.06 percent aluminum, ature. 0.01 to 0.12 percent columbium, 0.06 percent maxi- The low carbon content of the present composition mum cerium, the balance being iron and incidental as well. as acting to reduce pearlite permits all of the impurities. Depending upon the composition of the columbium required to provide the total strengthening steel, lower yield strengths in the range of 50,000 psi to effect to be dissolved or to be soluble in the austenite 80,000 psi are attained. This composition when hot thus assuring the maximum efficient use of the columrolled finished in a temperature range of l620 to bium. 1700F and coiled or collected in a temperature range The low carbon value together with the low mangaof 1l50 to 1375F, a unique relationship of strength nese value of the present composition act to increase and maximum formability of each of the yield strength the austenite to ferrite transformation temperature levels in the range exists. whereby the proper distribution of the columbium is At each strength level, from 50,000 to 80,000 psi, the attained and the columbium is enabled to carry out its final structure of the steel is composed principally of dual function, the coarse precipitation of columbium to ferrite with.very limited amounts of pearlite. In con.- provide grain refinement and the fine precipitation junction with this and essential to the improved formdispersion of columbium to provide strengthening after ability properties is the controlled dispersion of the transformation. The increase in transformation tempercolumbium as columbium carbides of columbium carature permits a higher final deformation or finishing bo-nitrides, as will be discussed later. While pearlite is temperature to effect grain refinement as discussed in grain boundaries and as cementite (Fe C) in the above and also permits a higher coiling temperature to form of skeletal carbides, the columbium has been effect precipitation strengthening. It is thus readily seen found to have a dual form, of row precipitates in excess that the amounts of carbon and manganese present in of 200 Angstrom units from which the initial ferrite the composition in combination with the finishing temgrains have formed and secondly, within the ferrite 5O perature and coiling temperature will permit the cograin itself, as a finely dispersed carbonitride in the lumbium to act effectively to provide the desired rerange of 30 to 120 Angstrom units. It may be noted in sults. V the steel composition of this invention that the level of I have found that for the strength level desired, a both C and Mn is considerably lower than known steels fairly precise amount of columbium content in combiin the same strength range which provides several nation with proper finishing and coiling temperatures major factors contributing to the improved formability an optimum relationship between strength and formproperties. The low content of the steel composition of ability will be attained. The following results of test the present invention, of course, reduces the pearlite work outlines the range of these parameters;

TABLE 1 Run N0. Lower Yield C Mn Cb Finishing Coiling Point Min) 7a 7c 7c Temp. "F Temp. 9F

p.s.i.

The prime consideration in obtaining maximum formability at each level of strength is related to the austenite to ferrite temperature which is controlled by the amount of C, Mn. Cb as well as the finishing and coiling temperatures. The temperature of coiling has been found to be most critical as such temperature establishes the final ferrite grain size and structure as well as the size and distribution of the columbium precipitate in the ferrite. The finishing temperature has Similarly thestrengthening effect may be calculated for Examples 2 to 4 and are summarized in the following also been found to be of major importance as such must Table ll TABLE II Example Average Basic Cb Average Total Strength- No. Strength Strength Strength- Cb Z: ening per 01% Level psi psi ening psi of Cb psi be low enough to Prevent recrystallization during the The overall relationship of strength attained from grain tlme p i the finish hot Work to the Start of ferrite refinement and precipitation strengthening is summattahstoimatloh and must be high enough to Produce rized in graphical form in FIG; 2. It may be noted that the largest possible equiaxial ferrite grain size to obtain precipitation Strengthening increases i proportion Optimum formabllltyfrom 20 percent to percent from Example No. l to Referehee may be had to the drawings Wherelhi Example No. 4. It may be further noted as shown in 1 Shows graphically the relationship between Table II that the total strengthening contribution of Cb columbium Content and the finishing temperature, through both grain size strengthening and precipitation 2 Shows graphically the relationship of Strength strengthening decreases per unit of Cb as the strength obtained from grain refinement and precipitation 30 increases Sttehgthehihg- In addition to the total strengthening contribution of From FIG. 1 an increase in columbium content in the Cb through b h grain i strengthening d i i composition acts to delay further recrystallization tion strengthening, the steels of the present invention thei a lowering of the columbium Content in the also have been found to have superior formability propposition or an increase in the finishing temperature erties at h strength l l Al h h there are many improves the risk Of recrystallization to normal austenparameters may be used to interpret and meaite from which a coarse grained ferrite is transformed. Sure f bilit two f h more i ifi t Ihave found that at a strength level of 50,000 psi, the ters are a) h fficie t f k h d i or Optimum tihlshihg temperature is lezool: t0 l67OF for valuesKstretchability) and b)'the coefficient of anisota columbium content of 0.015 percent. When a higher 40 ropy or values d bih y A i ll tlhlshlhg temperature is p y the Strength of the formed steel parts from hot rolled steel require both Steel drops p y when a higher Mn Content is p stretchability and formability a high value for both n eht, too g a finishing temperature Will require a and r is desirable. The method of determining both q n rate which Will Produce a non-equiaxial and n and rvalues is well documented in the literature deformed ferrite giaih- The higher finishing p thus no details with respect to these determinations are ture employed will guarantee a maximum columbium d d necessary precipitate in the ferrite as a percentage of the total I ddi i to h w d values, ddi i l available columbium parameters of the steels of the present invention were The ilhal Strength of the Steel Product is a hmetibh of also tested, namely uniform elongation e,, under unidisolld Solution Strengthening grain size Strengthening rectional strain'and the response to biaxial stretching as and Precipitation stiehgthehihgit y thus be y measured by gridded-cup samples (such test measureseen that where formability is a function of a coarse m b i ll known i h ny grain e and strength, to a large extent is a timetibh In order to attain optimum formability properties or of a title grain Size the Optimum ebmblhatibh Of forming values in the finished product close control on Strength and formablllty can y be attained y obtain certain factors during processing of the steel are neceslhg the maxlmum Strengthening Contribution from p sary. Although the presence of pearlite in a steel is a Cipitatiorl Strengthening. In Order to Clarify this, the economical manner of btaining good Strengthening etteet y be Stated in equation term for point and ultimate tensile strength properties of steel it example the general equation y be Stated asi is at the same time very detrimental to attaining good forming properties. Carbon of the steel of the present L.Y.S. (ksi) 15.0+4.72 (7zMn)+.5(D" )+AS invention is reduced to a low level of 0.10 percent wherein i lower Yield Strength maximum. It is maintained at this low level by the pres- Mn IS 7: manganese -0: is the ferme grain Size inter. ent invention by the use of the low manganese addition. F in inches y The low levels of both carbon and manganese reduce es gi fi z g fmm preclpmnon the formation of pearlite and thus contribute to in- For Example I creasing formability properties both directly and also Where a indirectly as control on the transformation temperature d' in 50.0 (ASTM l0.5)

of austenite to ferrite as previously discussed.

The steel composition employed should generally be free of both exogenous and indigenous inclusions of all types. During the deoxidation stage of the metal aluminum is added to ensure an accurate level of soluble The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of making a high strength low alloy steel aluminum in the finished product varying from 0.02 5 strip having a minimum yield strength of 50,000 psi and percent to 0.06 percent aluminum. It is recognized that good formability properties which comprises subjecting silicates are a major source of inclusion content which a slab of steel having a composition consisting essenmay result in deterioration of formability properties of tially of, by weight, 0.10 percent maximum carbon, the steel and for this reason silicon is not employed in 0.30 to 0.80 percent manganese 0.02 percent maxithe present invention as an alloy strengthener but is 10 mum sulphur, 0.02 to 0.06 percent aluminum, 0.01 to restricted as an incidental impurity, if present. The rare' 0.02 columbium, 0.06 percent maximum cerium, the earth metal of the composition is added as a sulphide balance being iron and incidental impurities, to a hotmodifier and also acts as a general cleaning agent by rolled finishing temperature in the range of l620to reducing other oxides which may be present. l670F and collecting the strip in a temperature range Another factor related to attaining optimum forming of 1325 to l375F. values resides in the development of the largest possi- 2. A method of making a high strength low alloy steel ble equiaxial ferrite grain size at each strength level strip having a minimum yield strength of 60,000 psi and desired. As previously indicated this is attained by obgood formability properties which comprises subjecting taining the maximum strengthening effect from the a slab of steel having a composition consisting essenoptimum columbium precipitate both coarse and fine tially of, by weight, 0.10 percent maximum carbon, in the ferrite through the use of the high finishing tem- 0.30 to 0.80 percent manganese, 0.02 percent maxiperature and the temperature at which coiling or colmum sulphur, 0.02 to 0.06 percent aluminum, 0.03 to lecting is effected. This is made possible through the 0.04 percent columbium, 0.06 percent maximum ceamount of Cb and Mn employed in the steel in conjuncriur'n, the balance being iron and incidental impurities, tion with the finishing and coiling temperatures. Table to a hot-rolled finishing temperature in the range of III illustrates the formability properties at the various l620to 1670F and collecting the strip in a temperastrength levels and the relationship of such properties ture range of 1250 to 1300F. to the composition and temperatures of processing. 3. A method of making a high strength low alloy steel TABLE III Run No. Strength Cb Finishing Coiling Grain Size Level Content Temp. Temp. ASTM e (Min) "F F n 7w r The finished products of the present invention were strip having a minimum yield strength of 70,000 psi subjected to a long series of stress-life fatigue tests and good formability properties which comprises subjecting results indicate that up to 2 X 10 cycles can be susa slab of steel having a composition consisting essentained with dynamic loads in excess of 40 percent of tially of, by weight, 0.10 percent maximum carbon, the ultimate tensile strengths. These results are be- 0.30 to 0.80 percent manganese, 0.02 percent maxilieved to be possible because of the high working hardmum sulphur, 0.02 to 0.06 percent aluminum, 0.06 to ening components of the steels which are a function of 0.08 percent columbium, 0.06 maximum cerium, the steel cleanness. balance being iron and incidental impurities, to a hot- The low carbon equivalent values of the steels of the rolled finishing temperature in the range of 1650F to present invention results in good weldability to both l700F and collecting the strip in a temperature range fusion and electric resistance techniques. The addition of l200 to 1250 F. of columbium in the steels in the range outlined, which 4. A method of making a high strength low alloy steel is present as the main strengthening element, has not strip having a minimum yield strength of 80,000 psi and been found harmful in the welding procedures. good formability properties which comprises subjecting In the tables above, reference is made to Run Numa slab of steel having a composition consisting essenbers 1 through 4. These are actual production runs tially of, by weight, 0.10 percent maximum carbon, made with the composition at the various levels of Cb. 0.30 to 0.80 percent manganese, 0.02 percent maxi- The strip or product was finished and coiled in the mum sulphur, 0.02 to 0.06 percent aluminum, 0.10 to temperature ranges specified in the table to attain the 0.12 percent columbium, 0.0 6 percent maximum cestrength levels of each run. It is believed that it is the rium, the balance being iron and incidental impurities, combination of the elements employed in the composito a hot-rolled finishing temperature in the range of tion with the restricted finishing and coiling tempera- 1650F to 1700F and collecting the strip in a temperatures used that maximum formability at each strength ture range of ll50 to l200F.

level is attained. 

1. A METHOD OF MAKING A HIGH STRENGTH LOW ALLOY STEEL STRIP HAVING A MINIMUM YIELD STRENGTH OF 50,000 PSI AND GOOD FORMABILITY PROPERITIES WHICH COMPRISES SUBJECTING A SLAB OF STEEL HAVING A COMPOSITION CONSISTING ESSENTIALLY OF, BY WEIGHT, 0.10 PERCENT MAXIMUM CARBON, 0.30 TO 0.80 PERCENT MANGANESE 0.02 PERCENT MAXIMUM SULPHUR, 0.06 PERCENT ALUMINUM, 0.01 TO 0L02 COLUMBIUM, 0.06 PERCENT MAXIMUM CERIUM, THE BALANCE BEING IRON AND INCIDENTAL IMPURITIES, TO A HOT-ROLLED FINISHING TEMPERATURE IN THE RANGE OF 1620* T0 1670*F AND COLLECTING THE STRIP IN A TEMPERATURE RANGE OF 1325* TO 1375*F.
 2. A method of making a high strength low alloy steel strip having a minimum yield strength of 60,000 psi and good formability properties which comprises subjecting a slab of steel having a composition consisting essentially of, by weight, 0.10 percent maximum carbon, 0.30 to 0.80 percent manganese, 0.02 percent maximum sulphur, 0.02 to 0.06 percent aluminum, 0.03 to 0.04 percent columbium, 0.06 percent maximum cerium, the balance being iron and incidental impurities, to a hot-rolled finishing temperature in the range of 1620*to 1670*F and collecting the strip in a temperature range of 1250* to 1300*F.
 3. A method of making a high strength low alloy steel strip having a minimum yield strength of 70,000 psi good formability properties which comprises subjecting a slab of steel having a composition consisting essentially of, by weight, 0.10 percent maximum carbon, 0.30 to 0.80 percent manganese, 0.02 percent maximum sulphur, 0.02 to 0.06 percent aluminum, 0.06 to 0.08 percent columbium, 0.06 maximum cerium, the balance being iron and incidental impurities, to a hot-rolled finishing temperature in the range of 1650*F to 1700*F and collecting the strip in a temperature range of 1200* to 1250 *F.
 4. A method of making a high strength low alloy steel strip having a minimum yield strength of 80,000 psi and good formability properties which comprises subjeCting a slab of steel having a composition consisting essentially of, by weight, 0.10 percent maximum carbon, 0.30 to 0.80 percent manganese, 0.02 percent maximum sulphur, 0.02 to 0.06 percent aluminum, 0.10 to 0.12 percent columbium, 0.06 percent maximum cerium, the balance being iron and incidental impurities, to a hot-rolled finishing temperature in the range of 1650*F to 1700*F and collecting the strip in a temperature range of 1150* to 1200*F. 